1. Field of the Invention
The invention relates to an electromagnetic high-pressure injection valve for injection of fuel into the combustion chamber of internal combustion engines.
The injection valve is intended principally for use in small and medium-sized diesel engines with a displacement of 300 cm.sup.3 to 700 cm.sup.3 per cylinder. The typical flow rate of the valve is 10-25 mm.sup.3 /ms. The injection valve can be used up to a fuel pressure of about 1000 bar. The valve has a needle-shaped valve-closing body which is connected to the armature of an electromagnet. As in the known low-pressure injection valves, the fuel feed to the injection nozzle is enabled when the armature is attracted. The injection valve is supplied with fuel by a high-pressure piston pump that is mechanically driven by the engine.
2. Description of the Related Art
In diesel engines, very high injection pressures of up to 1000 bar and above are aimed for in order to improve fuel preparation and to reduce the formation of pollutants. In general, a steep injection curve at the beginning of injection and a sharply delimited end to injection are demanded. The beginning and duration of injection must be matched to the conditions of the engine characteristic diagram.
Injection systems which are purely mechanical in operation are generally used for high-pressure injection. In this case the fuel is compressed at the beginning of the injection process in a pump element and the pump energy is transmitted as a pressure wave to the injection nozzle. The injection nozzle is provided with a nozzle needle which is lifted from the valve seat by the fuel pressure, counter to the force of a spring. In the case of small injection nozzles for vehicle engines, the mass of the nozzle needle is about 5-10 g. The return force of the spring is between 400 and 2000N, depending on the opening pressure of the nozzle. The seat diameter of the injection valves is in general about 2 mm. Due to the high return force and the relatively large mass of the nozzle needle, the valve seat is exposed to high impact loading upon closure of the valve.
During and after the injection process, powerful pressure waves are reflected between the pump and the nozzle. The amplitude of these waves can be up to several hundred bar. Given the pressure waves, touching of the zero line, during which the vapor pressure of the fuel is undershot, may occur after the closing of the injection nozzle. This leads to cavitation on the elements of the injection system, with high, shock-like loads. The reflected pressure waves can furthermore trigger a renewed needle opening process. During that process, an after-dribble delayed by the propagation time of the pressure wave occurs, during which the fuel is only inadequately atomized and only incompletely takes part in combustion. An additional after-dribble occurs due to the ever present needle rebound upon closure of the valve.
In the mechanically operating injection systems, the pumping process is rigidly coupled to a certain angle of rotation. High, shock-like mechanical loading of the injection pump occurs since the entire pressure build-up occurs in a very short time within the small angle of rotation. Since the time for passing through this angle becomes shorter and shorter as the engine speed increases, whereas the cross-section of the nozzle holes remains constant, a steep, speed-dependent pressure rise occurs, leading to considerable problems with fuel preparation. At low speeds, the pressure is usually insufficient to raise the nozzle needle completely. In the case of a partially opened needle, most of the fuel pressure is converted to speed in the valve seat and then swirled in the blind hole of the nozzle. Only a low fuel pressure is then available for speed conversion in front of the nozzle holes, resulting in very inadequate atomization.
The engine speed-dependent pressure rise makes it difficult to match the injection nozzle to the requirements of the engine, with the result that in the case of the mechanically operating injection systems, optimum conditions are achieved only in narrowly defined engine speed and load ranges. Injection valves with electromagnetic actuation can be used in order to avoid the problems resulting from the transportation of the fuel by pressure waves. In electromagnetic injection valves, a rapid actuating movement with little rebound is necessary to achieve sufficient accuracy of metering. That can only be achieved with an armature of very low mass with high mechanical rigidity. The attraction and release time should be less than 0.5 ms. The required short attraction time should be achieved with as little electric power as possible. The adaptation of the electromagnetic injection valves to the conditions of the engine characteristic diagram is simple to achieve with known electronic controls.
The known electromagnetic injection valves for the injection of fuel into the combustion chamber of internal combustion engines require a large magnetic force, which is required for overcoming the hydraulic forces acting on the valve needle. There are enormous difficulties in constructing sufficiently rapid electromagnets which can overcome the high hydraulic forces with tolerable energy expenditure. The known electromagnetic injection valves with a directly actuated valve needle have a very powerful electromagnet, which often has several simultaneously excited magnet coils. In order to achieve sufficiently rapid actuating movements with such an electromagnet, an enormous electric power must be made available for a short period of time. Furthermore, the armatures of such electromagnets are of as thin-walled construction as possible in order to obtain a low armature mass and in order to reduce eddy current formation in the magnet core. Due to the thin-walled construction, the armatures have a tendency toward pronounced mechanical vibrations in the case of rapid actuating movements, triggering unwanted rebound movements and disturbance forces.
An injection valve known from German Published, Non-Prosecuted Application DE-OS 23 43 243, which is provided for two-stroke engines, does not have any limitation for the injection stroke of the needle. That makes very precise control of the starting stroke necessary and the control is to be adjusted by manual means.